An air bag system is installed in many vehicles. The air bag system is installed not only to the driver's seat but also to the passenger's seat, which is next to the driver's seat. The air bag system inflates an air bag when it detects a collision of a vehicle with an object by means of an acceleration sensor in the vehicle.
The air bag system is generally set to inflate the air bag on the side of the driver's seat when the vehicle collides with an object, irrespective of whether a passenger is on the passenger's seat. In recent years, however, it is desired that the air bag system controls the air bag based on a physical size of a person on the seat. In the case that a grown person is sifting on the seat, the air bag system can protect the person by inflating the air bag when the vehicle collides with the object. However, in the case that a child is seated in a child restraint system fixed to the seat in a backward-looking posture, the air bag cannot protect the child effectively.
To cope with the problem, a passenger detection system is provided for determining whether a person in the passenger's seat is a grown person or a child in a child restraint system by means of a sensor at the passenger's seat. In JP patent 3346464B, an example of the passenger detection system is disclosed which has multiple antenna electrodes installed to the seat and detects by using a pair of the electrodes a passenger on the seat.
However, the passenger detection system in the JP 3346464B cannot detect whether the passenger on the seat is a grown person or not with a sufficient accuracy when the seat gets wet. This is because a wet portion of the seat functions as an antenna electrode and an output power from the electrodes on the seat increases. The increase of the output power may cause the passenger detection system to wrongly recognize the child in the child restraint system as a grown person. In this case, the air bag tends to expand incorrectly.
Another passenger detection system is disclosed in U.S. Pat. No. 6,816,077, US 2004-0196150A1 and US 2004-0199318A1 (all of them correspond to JP 2002-347498A) which has a water sensor and/or a humidity sensor and detects with a high accuracy whether the passenger on the seat is a grown person even when the seat is get wet. However, the sensor dedicated for detecting water on the seat increases manufacturing cost of the passenger detection system.